1. Field of the Invention
The present invention relates to a polishing apparatus and method, and more particularly to a polishing apparatus and method for polishing an object to be polished (substrate) such as a semiconductor wafer to a flat mirror finish.
2. Description of the Related Art
In recent years, high integration and high density in semiconductor device demands smaller and smaller wiring patterns or interconnections and also more and more interconnection layers. Multilayer interconnections in smaller circuits result in greater steps which reflect surface irregularities on lower interconnection layers. An increase in the number of interconnection layers makes film coating performance (step coverage) poor over stepped configurations of thin films. Therefore, better multilayer interconnections need to have the improved step coverage and proper surface planarization. Further, since the depth of focus of a photolithographic optical system is smaller with miniaturization of a photolithographic process, a surface of the semiconductor device needs to be planarized such that irregular steps on the surface of the semiconductor device will fall within the depth of focus.
Thus, in a manufacturing process of a semiconductor device, it increasingly becomes important to planarize a surface of the semiconductor device. One of the most important planarizing technologies is chemical mechanical polishing (CMP). Thus, there has been employed a chemical mechanical polishing apparatus for planarizing a surface of a semiconductor wafer. In the chemical mechanical polishing apparatus, while a polishing liquid containing abrasive particles such as silica (SiO2) therein is supplied onto a polishing surface such as a polishing pad, a substrate such as a semiconductor wafer is brought into sliding contact with the polishing surface, so that the substrate is polished.
This type of polishing apparatus includes a polishing table having a polishing surface formed by a polishing pad, and a substrate holding device, which is referred to as a top ring or a polishing head, for holding a substrate such as a semiconductor wafer. When a semiconductor wafer is polished with such a polishing apparatus, the semiconductor wafer is held and pressed against the polishing surface under a predetermined pressure by the substrate holding device. At this time, the polishing table and the substrate holding device are moved relative to each other to bring the semiconductor wafer into sliding contact with the polishing surface, so that the surface of the semiconductor wafer is polished to a flat mirror finish.
In such polishing apparatus, if the relative pressing force applied between the semiconductor wafer, being polished, and the polishing surface of the polishing pad is not uniform over the entire surface of the semiconductor wafer, then the surface of the semiconductor wafer is polished insufficiently or excessively in different regions thereof, which depends on the pressing force applied thereto. It has been customary to uniformize the pressing force applied to the semiconductor wafer by providing a pressure chamber formed by an elastic membrane at the lower portion of the substrate holding device and supplying the pressure chamber with a fluid such as air to press the semiconductor wafer under a fluid pressure through the elastic membrane, as seen in Japanese laid-open patent publication No. 2006-255851.
As described above, in the type of polishing apparatus which has a pressure chamber formed by an elastic membrane at the lower portion of the substrate holding device and supplies a pressurized fluid such as compressed air to the pressure chamber to press the semiconductor wafer under a fluid pressure through the elastic membrane, the following drawbacks have been discovered.
Specifically, after the semiconductor wafer is brought into contact with the polishing surface of the polishing pad, the semiconductor wafer is pressed against the polishing surface under the fluid pressure through the elastic membrane by supplying the pressurized fluid such as compressed air to the pressure chamber, thereby starting polishing of the semiconductor wafer. However, immediately after starting polishing of the semiconductor wafer, in some cases, there occurs a phenomenon that the semiconductor wafer is cracked or damaged.
The inventors of the present application have conducted various experiments and analyzed the experimental results for the purpose of finding out why the semiconductor wafer is cracked or damaged at the time of starting polishing of the semiconductor wafer. As a result, it has been discovered that some damage of the semiconductor wafer is caused by surface condition of the polishing pad. More specifically, it has been customary to form specific grooves or holes in the surface of the polishing pad. For example, there are a type of polishing pad which has a number of small holes having a diameter of 1 to 2 mm in the surface thereof to improve retention capacity of a slurry (polishing liquid), a type of polishing pad which has grooves in a lattice pattern, concentric pattern or spiral pattern in the surface thereof to improve fluidity of a slurry (polishing liquid), to improve flatness and uniformity of a surface of a wafer and to prevent a wafer from sticking to the surface of the polishing pad, and other types of polishing pads. In this case, in the type of polishing pad having no grooves, such as a polishing pad having small holes, in the surface thereof, or the type of polishing pad which does not have a sufficient number of grooves or sufficient depths of grooves in the surface thereof, it has been discovered that the semiconductor wafer is often cracked or damaged at the time of starting polishing of the semiconductor wafer after the semiconductor wafer is brought into contact with the surface (polishing surface) of the polishing pad.
The inventors of the present application have discovered from analysis of the experimental results that if the polishing pad has no grooves or does not have a sufficient number of grooves or sufficient depths of grooves in the surface thereof, then air or slurry is trapped between the polishing surface and the semiconductor wafer when the semiconductor wafer is brought into contact with the polishing surface, and thus the semiconductor wafer is likely to cause larger deformation than normal when polishing pressure is applied to the semiconductor wafer as it is, resulting in cracking or damage of the semiconductor wafer.
Further, as described above, in the type of polishing apparatus which has a pressure chamber formed by an elastic membrane at the lower portion of the substrate holding device and supplies a pressurized fluid such as compressed air to the pressure chamber to press the semiconductor wafer under a fluid pressure through the elastic membrane, there is a polishing apparatus which has a plurality of pressure chambers and can press a semiconductor wafer against a polishing surface under different pressures at respective areas in a radial direction of the semiconductor wafer by adjusting pressures of the pressurized fluid to be supplied to the respective pressure chambers. In this type of polishing apparatus, although the polishing rate within the surface of the semiconductor wafer can be controlled at the respective areas of the semiconductor wafer, because the holding and pressing surface of the polishing head for holding and pressing the semiconductor wafer comprises a flexible elastic membrane such as rubber, if there is a pressure difference in pressures of the pressurized fluid supplied to two adjacent areas, then there occurs a step-like difference in polishing pressures in the two adjacent areas. As a result, a step-like height difference in polishing configuration (polishing profile) is produced. In this case, if there is a large pressure difference in pressures of the pressurized fluid supplied to the two adjacent areas, the step-like height difference in polishing configuration (polishing profile) becomes larger depending on the pressure difference in pressures of the pressurized fluid supplied to the two adjacent areas.